Upper and lower casting belts in twin-belt continuous casting machines for continuously casting molten metal are relatively thin and wide. These casting belts are formed of suitable heat-conductive, flexible, metallic material as known in the art, for example such as quarter-hard low-carbon rolled sheet steel having a thickness for example usually in a range from about 0.045 of an inch to about 0.080 of an inch. These upper and lower belts are revolved under high tensile forces around a belt carriage in an oval path. During revolving in its oval path, each belt is repeatedly alternately passed around an entrance-pulley drum and an exit-pulley drum at respective entrance and exit ends of the moving-mold casting region in the machine.
The revolving upper and lower belts define a moving-mold casting region between them. This casting region is intended to be substantially defined between flat casting belts travelling from the entrance into the moving-mold region to the exit therefrom. Thus, the casting region is intended to extend from entrance to exit along a substantially flat casting plane.
The present invention deals with steering, tensioning and driving the revolving upper and lower casting belts. Therefore, to be more readily understood, this BACKGROUND will be set forth under three sub-headings:
Steering: As each highly-tensioned belt is revolving in its oval path, it inevitably tends to creep gradually edgewise in an unpredictable manner. Thus each belt must be steered individually. A belt cannot be steered by edge guidance efforts because edgewise creeping motion of a highly-tensioned, thin, metallic belt involves such large sideways (edgewise) forces that an edge of a revolving belt would crumple and tear against a futilely placed edge guide. Hence, each belt is steered by slightly tilting the axis of rotation of each exit-pulley drum. Entrance-pulley drums cannot be used for steering, because entrance-pulley drum axes must remain fixed so as to keep the mold entrance in a required predetermined cooperative relation with molten-metal infeed apparatus leading into the entrance.
Tilting-steering action of an exit-pulley drum currently is preferred to be accomplished by movements occurring in a plane which is substantially perpendicular to the casting plane.
A problem which occurs with tilting exit-pulley-drum axes by movements perpendicular to the casting plane is that such steering causes exit portions of each belt to become twisted slightly away from the casting plane. Consequenty, a newly cast slab loses support during critical moments while a downstream portion of this newly cast slab is moving along the casting region toward the exit end of the casting machine.
Tensioning: The upper and lower casting belts in a continuous casting machine wherein the belts are revolved in respective upper and lower oval paths are highly tensioned by exerting large forces for moving the axes of the upper and lower exit-pulley drums in a downstream direction. Entrance-pulley drums are not moved for tensioning purposes for reasons as already explained in regard to steering. Consequently, each belt is highly tensioned by moving the rotational axis of its exit-pulley drum by exerting large forces in a direction parallel with the casting plane for increasing slightly the distance between an exit-pulley drum and an entrance-pulley drum on the same carriage. This slight downstream movement of an exit-pulley drum continues the downstream movement required to take up the slack in a belt. Such slack is present in a newly-installed belt due to an upstream movement of an exit pulley which occurred previously to permit removal of a used belt and installation of a new belt onto the carriage.
Sometimes one edge of a casting belt is very slightly longer than the other, i.e., the belt when freely supported is very slightly frustroconical in configuration. Nevertheless, during continuous casting operation, the belt needs to be under substantially uniform high tension across the full width of the moving mold casting region.
Since each exit-pulley drum is being tilted for steering purposes in a plane substantially perpendicular to the casting plane, problems arise because this same drum also must be movable in a plane substantially parallel with the casting plane with large forces being applied in a direction substantially parallel with the casting plane for providing large tensile forces in the belt and wherein such tensile forces are substantially uniform across the full width of the casting cavity.
In certain prior-art machines as illustrated schematically in FIGS. 6A through 6F wherein there was a substantial neutral-position spacing of an exit-pulley drum from the casting plane P, as shown in FIGS. 6B and 6E, the forces involved during tilt-steering of a casting belt have caused significant diagonal stresses which in turn can cause diagonal fluting of the revolving belt. In practice, the high tensile forces involved in tilt-steering resulted in diagonal stresses in the flat reaches of the casting belt. Experience has shown that belts remain flatter, and a better product is cast, if the steering action can be minimized. Progress in this direction occurred with U.S. Pat. No. 4,940,076 of Desautels and Kaiser which disclosed a method and system achieving increased precision of steering, thereby minimizing the occurrences of and magnitudes (amplitudes) of steering motions. The method and the system invented by Desautels and Kaiser have been called "zero-point" belt position sensing and steering. However, the pattern of tilting of the exit-pulley drum in accord with their invention remained the same as occurred before their invention, namely, remained the same as shown in FIGS. 6A through 6C.
Belt-driving: During some recent years in continuous casting machines wherein the upper and lower casting belts are revolved in respective oval paths around entrance and exit-pulley drums, it had become usual practice to drive the revolvable casting belts by applying rotary driving force to the entrance-pulley drums. It had been preferred to drive the upper and lower entrance-pulley drums because the interiors of hollow exit-pulley drums were occupied by large "squaring shafts" (often being tubular "squaring tubes") of the prior art, rendering driving of those exit-pulley drums hardly feasible. Such squaring shafts were described in U.S. Pat. Nos. 3,949,805 and 3,963,068 of Hazelett, Wood and Carmichael, assigned to the same assignee as the present invention. Such prior-art squaring shafts were designed to ensure that the exit-pulley drums remained square with the carriage frames of the casting machine while these exit-pulley drums were being moved upstream and downstream in the direction parallel with the casting plane as described above.
A problem with revolving each belt around entrance and exit-pulley drums by rotatably driving its entrance-pulley drum arose from the fact that the belt was being pulled along its return (upstream) travel from exit to entrance. Conversely, during its downstream travel along the casting region, the driving force being applied to the belt by the rotatably driven entrance-pulley drum tended to reduce belt tension in areas of the belt immediately downstream from the entrance-pulley drum. These casting-belt areas near the entrance of the casting machine are very critical in the performance of a casting machine, because incoming molten metal flowed into the entrance is initially beginning to solidify against such belt areas. Initial solidification creates easily disturbed thin layers adjacent to the revolving casting belts. Undesired thermal belt distortions are more likely to occur in areas near the entrance where belt tension is reduced due to belt-driving force exerted by an entrance-pulley drum. Such thermal distortions may disturb and interfere with initial solidification of molten metal, thereby adversely affecting surface characteristics and/or overall qualities of a resultant continuously cast product.
Hence, it is desirable to drive the exit pulleys. Exit-pulley drive entails elimination of the prior-art squaring shafts from inside of the exit-pulley drums in order to permit attachment of a driving stub shaft to one end, the inboard end, of each exit-pulley drum for rotatably driving each exit-pulley drum. Also, a stub shaft is attached to the outboard end of each exit-pulley drum. The stub shafts projecting from each end of each exit-pulley drum serve as journals 63 and 64. Yet, the need for the "squaring" function remains.